I went on Tirerack.com to check out tires and tire/wheel combos since I am also interested in upgrading this summer. Here's some info I found out that might be useful:

1. When you put in our car (not the 94 XR2) it lists 185/60-14 as the OEM size. They do not sell 14" rims.2. The common tire/wheel combo size tire is 195/50-15 (OEM size for the 94 XR2). 3. For a 16" wheel, the tire size they list is 205/45-16.

I looked at a few tire/wheel combos using a 15" wheel. The offsets varied - 35, 40 and 45. The combos came with a centering ring. Is this ring needed if the offset is different from the OEM offset (which is what?)?

The offset determines how far the center of the wheel is moved from the hub face (how far the wheel tucks inside the fender, too close it will rub the strut)

The centering rings are if you wish to retain hubcentric wheels. The center of the stock wheel fits over a round boss on the hub, this centers the wheel & holds the load (as opposed to relying solely on the lugs for this)

Aftermarket wheels are generally made with much larger through-holes so they fit a wide variety of vehicles, and thusly you'll need an adapter ring to retain hubcentricity